History of the Universe - This diagram traces the evolution of the universe from the big bang to the present. Two watershed epochs are shown. Not long after the big bang, light from the first stars burned off a fog of cold hydrogen in a process called re-ionization. At a later epoch, quasars, the black-hole powered cores of active galaxies, pumped out enough ultraviolet light to re-ionize the primordial helium. Credit: NASA/ESA/STScI-A. Feild